come on powering on fans and fan controllers. pumps let alone the OC your going to be putting. *shiver*

Decent water cooling will result in cooler parts than air cooling, even with heavier overclocking. The cooler things are the less resistance, the less energy lost as heat. A transistor at 25C can be up to 200% more energy efficient than a transistor at 100C.

And fans and pumps are generally 5-15W each. Enough to counteract some of the power savings, but not all of them.

Adding water cooling will generally, depending on the system and implementation, result in the same power consumption +/-10%.